The present invention relates to a direct-current motor and a manufacturing method of the same.
A direct-current motor is generally used for driving wipers of a vehicle. One type of direct-current motors has three brushes to change the rotational speed of the motor. When the motor is in a normal operation, or running at a low speed, current is supplied to armature coils through a First brush and a second brush. When the motor is running at a high speed, current is supplied to the armature coils through the second brush and a third brush.
The direct-current motor includes a commutator, which is formed of a number of segments. The ends of each coil are connected to an adjacent pair of the segments, respectively The first, second, and third brushes slide along the commutator. While the motor is in a normal operation using the first and the second brushes, the flowing direction of the current through each coil is switched when a boundary of an adjacent pair of the segments, to which the coil is connected, passes each of the first and the second brushes. The switching of the flowing direction of current through each coil is referred to as the commutation. During the commutation, each of the first and the second brushes contacts an adjacent pair of the segments at the same time and establishes a short circuit in the coil connected to the pair of the segments. The period during which the coil is short-circuited is referred to as the commutation period.
When the third brush contacts an adjacent pair of the segments at the same time, while the direct-current motor is in a normal operation, the coil that is connected to the pair of the segments is short circuited through the third brush. At this time, great current instantly flows through the coil connected to the pair of the segments. The great current flows due to the induced electromotive force in the opposite direction to the flowing direction of current before the short circuit is established.
A graph in FIG. 26 shows the fluctuations of current that flows through one of the coils when the direct-current motor is in a normal operation using the first and the second brushes. Peaks A2, B2 of the current in the graph show abrupt changes in the current when the third brush establishes a short circuit between an adjacent pair of the segments to which the coil is connected. The third brush discharges sparks at the peaks A2, B2 of the current. The sparks produce noise and wear the third brush. Therefore, a member such as a coil (inductor) and a condenser need to be provided in the drive circuit of the motor for reducing noise. This increases the number of parts and the cost of the motor.
As shown by continuous lines C2, D2 in the graph of FIG. 26, when the commutation is performed in one of the coils while the motor is in the normal operation, the reactance voltage is generated in the coil. The reactance voltage delays change of the flowing direction of current. As a result, the current abruptly changes at the termination of the commutation period and the commutation is riot reliably performed.
The objective of the present invention is to provide a direct-current motor that suppresses the drawbacks caused by a short circuit, reliably performs commutation, and reduces the number of parts and cost. The present invention also provides a method for manufacturing the direct-current motor.
To achieve the foregoing objective, the present invention provides a direct-current motor including an armature, a commutator, a first, second, and third brushes, and a pair of magnets. The armature includes a core and a plurality of coils. The core has a plurality of teeth, which are spaced apart by a predetermined angular pitch. The teeth form a plurality of teeth groups, each of which includes a predetermined number of teeth. The tooth that is at the most advancing end in each teeth group in the rotation direction of the armature is the most advancing tooth. Each coil is wound about one of the teeth groups. The commutator integrally rotates with the armature. The commutator includes a plurality of segments. Each coil is connected to an adjacent pair of the segments. The first brush, the second brush, and the third brush are arranged about the rotational axis of the armature at predetermined angular intervals. The brushes can contact each segment to supply current to the coils through the segments. When current is supplied to the coils through the first brush and the second brush, the armature is rotated in a first mode. When current is supplied to the coils through the second brush and the third brush, the armature is rotated in a second mode. The pair of magnets face each other with the armature in between. One of the magnets has a magnetic flux change portion at a circumferential section of the magnet. When the third brush starts to establish a short circuit between an adjacent pair of the segments during the rotation of the armature, the most advancing end of the most advancing tooth in the teeth group that corresponds to the coil connected to the pair of the segments starts circumferentially overlapping the magnetic flux change portion. The circumferential dimension of the magnetic flux portion corresponds to the angle by which the armature rotates while the third brush establishes a short circuit between an adjacent pair of the segments.
The present invention also provides a direct-current motor including an armature, a commutator, a first, second and third brushes, and a pair of magnets. The armature includes a core and a plurality of coils. The core has a plurality of teeth, which are spaced apart by a predetermined angular pitch. The teeth form a plurality of teeth groups, each of which includes a predetermined number of teeth. The tooth that is at the most trailing end in each teeth group in the rotation direction of the armature is the most trailing tooth. Each coil is wound about one of the teeth groups. The commutator integrally rotates with the armature. The commutator includes a plurality of segments. Each coil is connected to an adjacent pair of the segments. The first brush, the second brush, and the third brush are arranged about the rotational axis of the armature at predetermined angular intervals. The brushes can contact each segment to supply current to the coils through the segments. When current is supplied to the coils through the first brush and the second brush, the armature is rotated in a first mode. When current is supplied to the coils through the second brush and the third brush, the armature is rotated in a second mode. The pair of magnets face each other with the armature in between. One of the magnets has a magnetic flux change portion at a circumferential section of the magnet. When the third brush starts to establish a short circuit between an adjacent pair of the segments during the rotation of the armature, the most trailing end of the most trailing tooth in the teeth group that corresponds to the coil connected to the pair of the segments starts circumferentially overlapping the magnetic flux change portion. The circumferential dimension of the magnetic flux portion corresponds to the angle by which the armature rotates while the third brush establishes a short circuit between an adjacent pair of the segments.
A further aspect of the present invention is a direct-current motor including an armature, a commutator, a first, second, and third brush, a pair of magnets, and a second magnetic flux change portion. The armature includes a core and a plurality of coils. The core has a plurality of teeth, which are spaced apart by a predetermined angular pitch. The teeth form a plurality of teeth groups, each of which includes a predetermined number of teeth. The tooth that is at the most advancing end in each teeth group in the rotation direction of the armature is the most advancing tooth. Each coil is wound about one of the teeth groups. The commutator integrally rotates with the armature. The commutator includes a plurality of segments. Each coil is connected to an adjacent pair of the segments. The first brush, the second brush, and the third brush are arranged about the rotational axis of the armature at predetermined angular intervals. The brushes can contact each segment to supply current to the coils through the segments. When current is supplied to the coils through the first brush and the second brush, the armature is rotated in a first mode. When current is supplied to the coils through the second brush and the third brush, the armature is rotated in a second mode. The pair of magnets face each other with the armature in between. Each magnet includes a main portion and an extended portion, which extends from the main portion. The extended portion includes a first magnetic flux change portion. The magnetic flux at the first magnetic flux change portion gradually increases toward the rotation direction of the armature. The second magnetic flux change portion extends circumferentially in a part of one of the main portions. When the third brush starts to establish a short circuit between an adjacent pair of the segments during the rotation of the armature, the most advancing end of the most advancing tooth in the teeth group that corresponds to the coil connected to the pair of the segments starts circumferentially overlapping the magnetic flux change portion.
The present invention also provides a direct-current motor including an armature, a commutator, a first, second, and third brush, a pair of magnets, and a second magnetic flux change portion. The armature includes a core and a plurality of coils. The core has a plurality of teeth, which are spaced apart by a predetermined angular pitch. The teeth form a plurality of teeth groups, each of which includes a predetermined number of teeth. The tooth that is at the most trailing end in each teeth group in the rotation direction of the armature is the most trailing tooth. Each coil is wound about one of the teeth groups. The commutator integrally rotates with the armature. The commutator includes a plurality of segments. Each coil is connected to an adjacent pair of the segments. The first brushy the second brush, and the third brush are arranged about the rotational axis of the armature at predetermined angular intervals. The brushes can contact each segment to supply current to the coils through the segments. When current is supplied to the coils through the first brush and the second brush, the armature is rotated in a first mode. When current is supplied to the coils through the second brush and the third brush, the armature is rotated in a second mode. The pair of magnets face each other with the armature in between. Each magnet includes a main portion and an extended portion, which extends from the main portion. The extended portion includes a first magnetic flux change portion. The magnetic flux at the first magnetic flux change portion gradually increases toward the rotation direction of the armature. The second magnetic flux change portion extends circumferentially in a part of one of the main portions. When the third brush starts to establish a short circuit between an adjacent pair of the segments during the rotation of the armature, the most trailing end of the most trailing tooth in the teeth group that corresponds to the coil connected to the pair of the segments starts circumferentially overlapping the magnetic flux change portion.
The present invention also provides a method for a direct-current motor. The motor includes an armature, a commutator, a first, second, and third brush, and a pair of magnets. The armature includes a core and a plurality of coils. The commutator integrally rotates with the armature. The commutator includes a plurality of segments. Each coil is connected to an adjacent pair of the segments. The first brush, the second brush, and the third brush are arranged about the rotational axis of the armature at predetermined angular intervals. The brushes can contact each segment to supply current to the coils through the segments. When current is supplied to the coils through the first brush and the second brush, the armature is rotated in a first mode. When current is supplied to the coils through the second brush and the third brush, the armature is rotated in a second mode. The pair of magnets face each other with the armature in between. The manufacturing method includes a first polarization process and a second polarization process. In the first polarization process, the magnets are exposed to a part of a magnetic field having a generally uniform direction and a generally uniform force, so that the magnets have the magnetic flux having a generally uniform direction and a generally uniform force. In the second polarization process, a part of one of the magnets is exposed to a part of a magnetic field having a generally opposite direction to the magnetic field generated in the first polarization process, thus forming a magnetic flux change portion at a circumferential section of the magnet. The magnetic flux generated in the magnetic flux change portion suppresses the electromotive force induced in the coil due to a short circuit established between an adjacent pair of the segments by the third brush during the rotation of the armature in the first mode.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.